(1) Field of the Invention
This invention relates generally to the field of audio amplifiers and relates more specifically to over-current protection of integrated class D audio amplifiers.
(2) Description of the Prior Art
Over-current protection requirement:
Most applications require to bound output signals within some defined limits. Over-current protection is required to protect an integrated switch-mode audio power amplifier against high currents that can flow through the output devices. In switched mode audio amplifiers large output devices used for high power efficiency offer a low resistance path between the power supply (battery) and ground via load. Unbound currents can damage output devices. The damages caused by high currents inside the IC are mainly due to electro-migration in metallization tracks and bond wires. The limited power dissipation due to large DC currents within MOS devices can also be an issue.
Prior art operation:
Provided the outputs of a class-D audio amplifier (H-bridge tied load) are connected via a load (Rload>4 Ohms) and the voltage supplies remain within specification even a full scale pulse-width modulated (PWM) signal, for example 95% modulation, will not result in a output current that significantly exceeds maximum specified output current (safe maximum for output devices/metal tracks/bond-wiring). The output current Io amounts to:
            I      o        =                  M        ×        Vbat                    Rload        +        Rsw        +        Rconnect              ,wherein M is a modulation index, Vbat is a power supply voltage (e.g. a battery), R (load, sw, connect) is an impedance of load, switch device, and connect impedance.
In order to give an example of an over-current situation it has to be noted that nothing physically prevents the input's amplitude to exceed a specified range. In case the amplifier's gain is set to e.g. 6 dB, a feedback system will try to reproduce the large amplitude at the output amplified by a factor of 2. In such case, the output signal of a loop filter stage will exceed maximum or minimum values of the triangular PWM waveform causing comparators that produce the PWM signal to become saturated either one or zero for a period of time that is defined by the input signal. The PWM output stream becomes over-modulated and the outputs are consequently fixed in one state (fully ON or OFF). This produces a DC path from supply to ground via the load. Where this is the case, the current flows through the output devices according to Ohms law and a 6V supply can produce almost 1.5 A through a 4 Ohms load.
Solutions dealing with over-current detection schemes are described in following articles/patents:
Marco Berkhout describes in his article “Integrated Overcurrent Protection system for class-D audio power amplifiers” (IEEE JSSC Vol. 40, no. 11, November 2005) a series detection scheme and a parallel detection scheme:
1. Series detection scheme:
A small “sense” resistor is inserted between the power supply pin and the source of the output device. A comparator compares the voltage drop across the resistor against a reference to detect an over-current situation. The drawbacks of such a detection scheme are the following: The resistance inserted in the power path decreases efficiency, a high precision comparator and reference voltage are required, and process spread and temperature dependence of metal resistance cause inaccuracies.
2. Parallel detection
The current through the power transistor is measured indirectly by comparing its voltage drop across an output device (Vds) using the voltage drop of a N-times smaller replica transistor (parallel device with same Vgs) that is biased at a reference current. The drawbacks are current consumption and an additional chip-area required, which determines the scheme accuracy through device matching). Accuracy means low N factor that in return means large chip area and large reference current.
Furthermore Santillano (U.S. Pat. No. 6,339,360) discloses
3. Digital detection
When the input of comparators exceeds a reference triangle waveform, the pulse-width-modulated signal output by the comparators gets saturated (either high or low). A digital counter (or any other digital detection circuit) detects that the PWM is “locked” in either of these two states and inserts narrow pulses to lower the modulation index, thus lowering the current at the same time. Saturation implies that the modulation index is 100%.
Moreover the following patents deal with over-current protection:
U.S. Patent Application Publication (US 2009/0153251 to Cheng et al.) discloses a voltage detection type over current protection device, which applies to the output stage of a CMOS Class-D audio amplifier. Generally, a Class-D audio amplifier is used to drive a high-load loudspeaker; therefore, it needs a high-current driver. When there is a short circuit in the load, the high current will burn out the driver stage. The present invention detects the output voltage to indirectly monitor whether the output current is too large. Once an over current is detected, the output-stage transistor is turned off to stop high current lest the circuit be burned out.
U.S. Patent Application Publication (US 2009/0160428 to Tai) proposes an over current detection device, which uses a first NOT gate and a second NOT gate to reverse the logic states of a first digital signal and a second digital signal which are digitalized audio signals in a class D power amplifier. Next, a CMOS transistor receives the reversed digital signals and drives a load. A comparing circuit detects the current of the load and compares the current with the reversed first and second digital signals. When the current of the load is too high, the comparing circuit respectively outputs a first electrical signal and a second electrical signal to a first logic gate and a second logic gate. Then, the logic gate outputs a signal to activate a protection circuit to prevent the entire circuit be damaged or burned out.
U.S. Patent Application (U.S. Pat. No. 7,554,409 to Zhang et al.) proposes an over-current protection circuit and method for protecting switching power amplifier circuits providing protection against latch-up and other failures due to energy returned from an inductive load when one or more transistors in the amplifier output are disabled in response to an over-current condition. Upon detection of an over-current condition, the transistor corresponding to the over-current conduction direction is disabled. At the same time, the transistor corresponding to the conduction direction opposite the over-current direction is enabled for a predetermined time period, or until the magnitude of the load current has dropped, so that energy stored in inductance of the load is reduced, preventing back-currents that would otherwise cause latch-up and consequent destruction of the output stage when the switching power output stage is disabled. After the predetermined time period has elapsed or the load current has dropped below a threshold, the entire output stage is disabled.
U.S. Patent Application (U.S. Pat. No. 5,917,369 to Nguyen) discloses a pulse-width modulation (PWM) circuit in a Class D audio amplifier including output-limiting logic and an automatic gain control (AGC) circuit. When an out-of-range, or overmodulated, input signal is received by the PWM, mono-stable multivibrator circuits provide discharge pulses that ensure that the PWM output will not spend excessive time in a single state. By using discrete mono-stable multivibrators, uniform and repeatable pulses can be generated at precise intervals.
Furthermore U.S. Patent Application (U.S. Pat. No. 6,917,569 to Nguyen) discloses a Class D amplifier including a tracking circuit and a regulation circuit to provide efficient, accurate protection against excessive current flow in the power FET's of the amplifier output stage. The FET currents flowing in the output stage are mirrored across tracking resistors by the tracking circuit. The resultant voltage drops are compared to reference voltages to detect over-current situations. By mirroring scaled-down versions of the output currents, power consumption in the tracking circuit is minimized. When an over-current situation is detected, the regulation circuit modulates the duty cycle of the signal input to the output stage until the FET currents fall to acceptable levels.
U.S. Patent Application Publication (US 2008/0218152 to Bo) discloses a method for sensing the supply current of a switched DC-to-DC converter. The method sensing a first voltage that is proportional to the supply current, wherein the first voltage has first noise; outputting a second voltage that is based on the first voltage, and wherein the second voltage has second noise that is smaller than the first noise; and comparing the second voltage to a reference voltage to provide an indication of the supply current. According to the systems and methods disclosed herein, accurate current sensing is provided.